1. Field of the Invention
The present invention relates to an auto emission purification catalyst composed of palladium on an .alpha.-alumina support. More particularly, the present invention relates to an auto emission purification catalyst of palladium on an .alpha.-alumina carrier having special characteristics which diminish the tendency to oxidize SO.sub.2 to SO.sub.3 in the exhaust gases while effectively oxidizing unburned hydrocarbons and CO in the exhaust gases.
2. Description of the Prior Art
The gasoline which is normally used in automobiles contains residual sulfur compounds and has a sulfur content of less than 0.05 weight %. The greater portion of these compounds reacts with oxygen in the burning process of the combustible gas-air mixture in the cylinder, thereby forming sulfur dioxide as a pollutant. The sulfur dioxide is expelled from the engine in the exhaust gas and in the process is widely diffused into the atmosphere where it is converted into sulfur trioxide via photochemical reactions. Thereafter, hydrolysis of the SO.sub.3 results in sulfuric acid mist which causes severe environmental problems.
In order to meet the ever increasing stringent requirements of auto emission control laws, noble metal based catalyst converters have been used to eliminated unburned hydrocarbons and carbon monoxide from the exhaust gases expelled from vehicles. However, the purification of the exhaust gases in such catalyst converters results in the oxidation of unburned hydrocarbons and carbon monoxide, which promotes the transformation of sulfur dioxide into sulfur trioxide, thereby producing sulfate dust and sulfuric acid mist in the auto exhaust system. Thus, the exhaust gas emissions contain sulfuric acid mist and sulfate dust, and the problem of the catalyst converter-equipped vehicles is how to reduce these sulfur pollutants in the auto emissions.
Alumina, which is the catalyst carrier, reacts with sulfur trioxide when the temperature of the catalyst bed is low and occludes sulfur trioxide, but at high temperatures the occluded sulfur trioxide is released. Thus, while the temperature of the catalyst bed is low when the engine is idled or under low speed running conditions, a large volume of sulfur trioxide is generated. Moreover, the greater portion of the sulfur trioxide generated under these conditions combines with the alumina support yielding Al.sub.2 (SO.sub.4).sub.3 which is occluded in the porous structure of the carrier and thus reduces the volume of SO.sub.3 discharged from the engine.
The above reaction progresses easily in the vicinity of 420.degree. C., but when the temperature rises to about 700.degree. C., Al.sub.2 (SO.sub.4).sub.3 is decomposed. Thus, a large volume of SO.sub.3 is produced under low-speed, low-load running conditions when the temperature of the catalyst bed rises, resulting in a sudden release of SO.sub.3 in large volume and causes local environmental pollution.
Accordingly, a need continues to exist for a method of reducing the sulfur oxide content of exhaust gases.